Printed circuit boards are typically tested using a device consisting of an array of pogo pins. The device is positioned on the board so that the pogo pins contact leads of the components on the printed circuit board. The circuit board is then tested by measuring electric current through the pogo pins. FIG. 1 shows one example of a commonly used pogo pin. This probe is composed of two subassemblies, a socket (47), and a moveable probe assembly (48) which is inserted into the socket (47) to form a complete pogo pin assembly (51). The pogo pin is used to deliver repeated electrical access to the Printed Circuit Board (PCB) under test. The socket (47) provides 2 functions: it allows the probe assembly (48) to be easily replaced after wear or damage, and it prevents potential damage to the delicate probe assembly (48) on insertion of the socket (47) into the alignment plate 41 (FIG. 2).
The probe assembly (48) includes a probe body (49), a probe spring (50) and a probe tip (46). The socket assembly (47) includes a socket body (44) and a socket tail (43). The probe tip (46) contacts a test pad on the PCB when the PCB is compressed onto the pogo pin tip. To complete the electrical connection between the PCB and the test system, the socket's tail (43) contacts a translation board as the fixture interface or alternatively is wire wrapped to the fixture interface.
The socket is forced into a thick plate of material (41) in order to be held in place during use (FIG. 2). The plate (41) also serves to counteract the force used to compress the PCB under test on to the pogo pins.
A typical application (e.g., PC motherboard) requires in excess of 1000 pogo pins. A standard 0.025" probe would use a spring with 4 ounces of force at 2/3 compression. In a PC motherboard application, the thick plate of material (41) would have to support a pressure of over 250 pounds. In order to counteract such a high force without deformation, a very thick fiberglass and glass composite is utilized for the plate material. This material is generally referred to as "G-10". This material is very thick, averaging about one half inch, and very difficult to drill straight holes for socket 47 (FIG. 1) insertion. If the holes are not straight, the assembled pogo pin will not be perpendicular to the PCB. In this case, the probe may miss its target and will succumb to excessive wear and early life failure.
The drill bit's strength is limited by their size and their ability to maintain a precisely perpendicular hole is impacted by the thickness and length of the drill and the consistency of the material being drilled. G-10, the material (41) used to hold the probe socket 47 in place, is a molded material that uses inconsistent fiber sizes within. The strength, thickness and inconsistency of the material make it very difficult to drill straight. In an attempt to overcome this problem many different drill processes have been experimented with and several are in use today. The more consistent processes require drilling shallow holes from each end side of the material. This reduces the distance the drill bit can drift from the desired position. It also adds cost and increases the time required to complete the fixture.
Even thick G-10 material cannot resist the constant pressure from the probes. Over time, the G-10 material deforms and the socket deviates from it's perpendicular position. Replacement of pogo pins causes stress on the socket which can further affect perpendicularity.